Most comedians create a screen persona that lasts a lifetime or they use a version of themselves. A few steadily assemble a formidable gallery of notable figures while retaining a personal identity known only to a circle of friends. The two most notable actors in the second category are Alec Guinness and Peter Sellers, and it is in their company that Sacha Baron Cohen belongs, though he has yet to make his Kind Hearts and Coronets or Dr Strangelove. Cohen has turned in some creditable performances, each sporting an outrageous foreign accent, in what are essentially other people's films: the Italian pedlar of phoney elixirs in Tim Burton's Sweeney Todd; the gay French grand prix driver in the Will Ferrell vehicle Talladega Nights; and the Parisian cop reminiscent of Sellers in Scorsese's Hugo.

It is, however, for the central performances in works centring on characters of his own creation that he is most celebrated, in which he also speaks a wild, fractured English – Ali G, the British homeboy of dubious background; Borat, the TV reporter from Kazakhstan; Brüno, the Austrian fashion writer; and now Admiral General Aladeen, the Middle Eastern tyrant ruling the oil-rich desert state of Wadiya in The Dictator.

These last four figures are intended to shock and outrage by their defiance of conventional propriety and current standards of decency and political correctness. One way and another, and for a variety of purposes, part of the joke is that they act out of naivety and ignorance rather than calculated malevolence. In the case of Ali G, Borat and Brüno, they engage spontaneously with non-actors who believe them to be real people, usually as a way of exposing prejudice and concealed feelings, though in some cases what is actually revealed is kindness, tolerance or an honest bewilderment. In this form of guerrilla cinema, the performer courts real danger that, in its extreme form, might stop just short of actual lynching. Aladeen, on the other hand, is a truly dangerous person, a combination of Gaddafi and Saddam Hussein, but the film he appears in is a conventional comedy, entirely scripted. Playing him poses no real danger for Baron Cohen other than his possible rejection by audiences, who might refrain from laughing, or by moralists who believe his bad taste goes beyond acceptable limits.

The first thing to be said, therefore, is that the movie is full of jokes and they're scattered like a fistful of seeds sown by a frantic farm labourer. Some inevitably fall on stony ground. But as with Mel Brooks, it's the good scenes and lines that stick in the mind rather than the misfires. The first joke that comes to mind is the last one in the movie and it occurs during the credits as most of the audience is heading for the exits. Aladeen's latest addition to his harem informs him that she's pregnant and he casually asks: "Are you having a boy or an abortion?"

The second thing is that the movie is curiously inoffensive. This in itself is oddly offensive as the character we're dealing with is a genocidal monster, indifferent to the sufferings of his people and in the process of manufacturing a weapon of mass destruction that he intends to drop on a neighbouring state, though he stops halfway through naming Israel as the recipient and suppresses a chuckle. One of the reasons for the benign response it elicits is that Baron Cohen presents himself as a good sport. We've seen him on TV walking along the red carpet at the Oscar ceremonies dressed as the bemedalled Aladeen and making fun of the solemn proceedings. Instead of having the customary press screening, the distributors compelled critics in London to attend the premiere at the Royal Festival Hall where Aladeen arrived surrounded by uniformed girls in minidresses described as his Virgin Guard and proceeded to address the audience from a VIP box. More significantly perhaps, after the film has shown Aladeen order the deaths of numerous people who've annoyed him, it's revealed as an additional joke that they've all managed to escape execution. This is the precise opposite of wiping the smile off your face.

The ramshackle plot is principally a hat stand for the jokes, which take in everything from masturbation to a misunderstanding over the role of a rape centre and has Aladeen visiting New York to win international favour by addressing the United Nations. A would-be usurper in the Wadiya government (played by an ill-at-ease Ben Kingsley) arranges an assassination attempt. This ends with a dim-witted double replacing the dictator, and the temporarily deposed Aladeen having his trademark beard shaved off. Presumed dead, he's protected by the feminist owner of an organic food shop (Anna Faris), a major butt for his misogyny. It's essentially a version of Mark Twain's The Prince and the Pauper (which was more adroitly reworked as Trading Places). But it also appears to draw on Chaplin's The Great Dictator, where the Hitlerian tyrant Hynkel is replaced by his double, Chaplin's Tramp. Indeed at one point a self-pitying Aladeen refers to himself as the last of the great dictators.

The film's most successful shot at true satire is the precise equivalent of Chaplin's deadly serious "Look up, Hannah" speech in The Great Dictator. In Baron Cohen's film, Aladeen makes a double-edged speech to a gathering of politicians and diplomats urging America to abandon democracy, listing a succession of things they would thus avoid, all of them familiar American shortcomings and inequities. This got by far the biggest laugh of the evening from the audience I saw it with, and I laughed along with them.